1. Field of the Invention
The present invention relates to a high-voltage stabilizer circuit suitable for use in a monitor. More particularly, the present invention relates to a high-voltage stabilizer circuit for a monitor which stabilizes the output voltage of a high-voltage generating circuit by preventing an abrupt overshoot of the output voltage which may occur when the power is initially turned on.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a conventional high-voltage generating circuit. The conventional high-voltage circuit is provided with a flyback transformer T1 for generating a high voltage in accordance with a direct current (DC) voltage outputted from an input voltage adjustment section 4, a high-voltage sensing section 1 for sensing the high voltage induced in the secondary winding of the flyback transformer T1 and outputted through a high-voltage generating section 6, an amplifying section 2 for inverse-amplifying the high voltage sensed by the high-voltage sensing section 1, and a pulse-width modulation integrated circuit (PWM IC) 3 for pulse-width modulating the high voltage provided from the amplifying section 2 in accordance with an external horizontal synchronous signal and a voltage fed back from the input voltage adjustment section 4. The conventional high-voltage circuit is also provided with the input voltage adjustment section 4 for controlling the voltage value inputted to the flyback transformer T1 in accordance with a pulse signal provided from the PWM IC 3, and a horizontal output circuit section 5 for providing to the flyback transformer T1 a control voltage for keeping the output voltage of the input voltage adjustment section 4 constant.
The high-voltage sensing section 1 is composed of a resistor R3 for sensing the high voltage induced in the secondary winding of the flyback transformer T1, and a capacitor C2 for smoothing the sensed voltage.
The amplifying section 2 includes an amplifier A1 for amplifying the high voltage sensed by the high-voltage sensing section 1 with a predetermined amplification factor to provide the amplified output signal to the PWM IC 3, and a variable resistor VR1 for variably determining the amplification factor of the amplifier A1.
The input voltage adjustment section 4 includes transistors Q1 and Q2 performing switching operation in accordance with the pulse-width-modulated signal outputted from the PWM IC 3 to provide a switching control signal, and a transistor Q3 which is turned on or off in accordance with the switching control signal to provide a prescribed voltage inputted thereto to the primary winding of the flyback transformer T1 as well as to the PWM IC 3 as a feedback signal.
The horizontal output circuit section 5 comprises a horizontal output transistor Q4 which performs switching operation in accordance with the pulse signal supplied from an external horizontal drive circuit to provide a pulsed voltage to the primary winding of the flyback transformer T1.
The reference numerals L1 to L3 denote coils, R1, R2, R4 to R8 denote resistors, and C1, C3 to C7 denote capacitors. The reference numerals D1 and D2 denote diodes, and ZD1 denotes a Zener diode.
The operation of the conventional high-voltage circuit as constructed above will be described with reference to FIGS. 1 and 2.
When the power of the monitor is on, and a predetermined voltage is supplied to the primary winding of the flyback transformer T1, a high voltage is induced in the secondary winding of the flyback transformer T1.
At this time, the resistor R3 in the high-voltage sensing section 1 senses the high voltage induced in the secondary winding of the flyback transformer T1, and provides a voltage shown as "A" in FIG. 2 to the inverting terminal of the amplifier A1 in the amplifying section 2. The amplifier A1 inverse-amplifies the voltage as shown as "A" in FIG. 2, which is sensed by the resistor R3, smoothed by the capacitor C2, and then inputted to the amplifier A1 through the resistor R4, with an amplification factor determined by the variable resistor VR1, and output the inverse-amplified voltage as shown as "B" in FIG. 2 to the PWM IC 3 through the resistor R6 and the Zener diode ZD1. The inverse-amplified voltage outputted from the amplifier A1 will be used as a reference voltage for the PWM operation of the PWM IC 3.
The PWM IC 3 pulse-width-modulates the voltage outputted from the amplifier A1 as shown as "B" in FIG. 2 in accordance with the external horizontal synchronous signal, and outputs a pulse signal as shown as "C" in FIG. 2 to the transistors Q1 and Q2 in the input voltage adjustment section 4.
The transistor Q3 performs switching operation in accordance with the pulse signal outputted from the transistors Q1 and Q2, and chops the prescribed voltage V1 provided through the coil L1. The transistor Q3 provides the chopped voltage to the primary winding of the flyback transformer T1 through the diode D1 and the capacitor C5, and simultaneously feeds back a saw-tooth voltage as shown as "D" in FIG. 2 to the PWM IC 3 through the resistor R7 and the capacitor C4. The saw-tooth feedback signal is obtained by integrating a square wave signal as shown as "C" in FIG. 2 through the resistor R7 and the capacitor C4.
When the voltage chopped by the switching operation of the transistor Q3 is supplied to the primary winding of the flyback transformer T1, a high voltage is induced in the secondary winding of the flyback transformer T1, and the induced high voltage is inputted to the PWM IC 3 through the high-voltage sensing section 1 and the amplifying section 2. The PWM IC 3 pulse-width-modulates the high voltage outputted from the amplifying section 2 in accordance with the voltage fed back by the transistor Q3 in the input voltage adjustment section 4 and the external horizontal synchronous signal, and provides the pulse-width-modulated voltage to the transistor Q3 through the transistors Q1 and Q2 in the input voltage adjustment section 4, keeping the voltage outputted from the input voltage adjustment section 4 constant. The transistor Q3 performs switching operation in accordance with the pules signal from the PWM IC 3 to keep the voltage inputted to the flyback transformer T1 as shown as "E" in FIG. 2.
At the same time, the horizontal output transistor Q4 in the horizontal output circuit section 5 performs a switching operation in accordance with the horizontal drive signal outputted from the horizontal drive section to provide a predetermined voltage to a horizontal deflection coil.
As described above, the conventional high-voltage generating circuit enables a constant voltage to be induced in the secondary winding of the flyback transformer T1 by sensing the high voltage induced in the secondary winding of the flyback transformer T1 and then by controlling the voltage provided from the input voltage adjustment section 4 to the flyback transformer T1 with the help of the amplification operation of the amplifying section 2 and the control operation of the PWM IC 3.
According to the conventional high-voltage generating circuit, however, the time T1 when the high voltage induced in the secondary winding of the flyback transformer T1 reaches a steady state is lengthened as shown as "A" in FIG. 2 due to the time constant of the internal resistors R1 and R2 and capacitor C1 of the flyback transformer T1, and according to this, the voltage amplified by the amplifying section 2 during the time period has a big overshoot as shown as "B" in FIG. 2
Specifically, the conventional high-voltage circuit has the disadvantages that the initial voltage provided to the flyback transformer T1 through the high-voltage sensing section 1, amplifying section 2, PWM IC 3, and input voltage adjustment section 4 has an abrupt rise, and this causes an excessive voltage to be applied to the horizontal output transistor Q4 in the horizontal output circuit section 5, resulting in overload on the horizontal output transistor Q4.